Electrode and secondary battery using the same

ABSTRACT

The invention relates to an electrode suitable for a chargeable/dischargeable secondary battery in which a carbonaceous material capable of doping and dedoping of lithium ions is used as a negative electrode active material, wherein carbon fibers are used as the carbonaceous material in a form of an uni-directionally arranged body or in combination of electrically conductive foil or fibers, and further relates to a secondary battery using the electrode. The invention enables to provide a secondary battery having high capacitance and high outputtng property.

TECHNICAL FIELD

The present invention relates to an electrode using carbon fibers and achargeable/dischargeable secondary battery using the same.

BACKGROUND ART

In recent years, small secondary batteries having high capacitance havebeen remarkably demanded with the spread of portable devices such asvideo cameras and notebook-type personal computers. Most of thesecondary batteries currently used are nickel-cadmium batteries whichuse alkaline electrolytic solutions. Such secondary batteries, however,show low battery voltages of about 1.2 V, and therefore are difficult tobe improved in energy density. Under these circumstances, it has beeninvestigated the high energy-type secondary batteries using lithiummetal, which is the basest metal, for negative electrode.

However, the secondary batteries in which lithium metal is used fornegative electrode have disadvantages such that the lithium develops todendrites by the (re)charging/discharging cycle, which may cause a shortcircuit and further cause the danger of ignition of the batteries. Inaddition, as lithium metal used in a secondary battery is very active,such a battery itself involves highly dangerous factors. Therefore, theyare questionable in domestic applicability. In order to solve theproblems relating to safety described above, lithium ion secondarybatteries using various carbonaceous materials have been proposedrecently, by which high energy inherent to the lithium electrode can begiven. The secondary batteries of this type are devised by utilizing thephenomenon that, since the carbonaceous material doped with lithium ionsat charging comes to have the same electric potential as metal lithium,the carbonaceous material doped with lithium ions can be used fornegative electrode in place of metal lithium. In this type of secondarybattery, when discharged, the lithium ions which have been doped to thecarbonaceous material are dedoped from the negative electrode and goback to the carbonaceous material to which the lithium ions have beendoped originally. Therefore, the use of carbonaceous material doped withlithium ions for negative electrode never causes the problem of dendriteproduction, and furthermore gives excellent safety since metal lithiumis not present; therefore has now been investigated extensively.

As the secondary batteries utilizing the doping of lithium ions tocarbonaceous material, those have been known, for example, disclosed inJapanese Patent Application Laid-open Nos. 90863/1987 and 122066/1987.The carbonaceous materials used in the references above are generally ina form of powder, and therefore is required to be incorporated with apolymer as a binder such as Teflon and poly(vinylidene fluoride) formolding into an electrode. That is, an electrode can be prepared in themanner that a powdery carbonaceous material is mixed with a binder andthen adhered to a metal mesh, or applied on a metal foil as a slurry. Onthe contrary, as for carbon fibers, there has been no precedent in whichcarbon fibers are practically used for electrodes of secondary batteriesindustrially. Therefore, the form or structure of electrode to bepreferably employed or the preparation technique of such electrode hasbeen quite unknown. In particular, the most serious technical problemsare how to shape carbon fibers into an electrode, how to take theelectrical contact of the carbon fiber with a current collector, how tosolve a problem of electrical short circuit between a positive electrodeand a negative electrode caused by the penetration of fluffs of thecarbon fibers through a separator, and so on.

However, when carbon fibers are used in a form of non-woven fabric orwoven fabric, the electrode can be prepared without or, if any, a traceamount of a binder. In addition, it is recognized that the use of carbonfibers for electrode is excellent with respect to chemical stabilityagainst electrolytes, structural stability against volume expansioncaused by doping, cyclicity of (re)charging and discharging, and so on.As the secondary batteries using such electrodes, those have been known,for example, disclosed in Japanese Patent Application Laid-open Nos.54181/1985 and 103991/1987. The electrodes using carbon fibers asdescribed above, however, have a defect that the electrical connectionwith a metal, i.e. a taking-out electrode, becomes difficult. In case ofa carbon powder electrode, since the carbon powder is mixed with abinder and then adhered to a metal mesh or applied on a metal foil as aslurry as described above, the metal mesh or metal foil can be used as acollecting electrode for the connection with a terminal. On thecontrary, in case of a carbon fiber electrode, it has been tried toinsert the ends of the carbon fibers into a mesh-shaped or foil-shapedcurrent collecting metal electrode to be fixed. However, the carbonfiber tend to become into pieces and to be broken easily, andconsequently the use of the carbon fibers is still disadvantageous inworkability in preparation of electrode, as well as mechanical strengthand durability of the resulting electrode. Further, such problem alsooccurs that the fluffs, i.e. broken fibers, penetrate through aseparator to cause the electrical contact between a positive electrodeand a negative electrode, which resulting in an internal short circuit.Furthermore, such problem occurs that, since the carbon fibers aremerely inserted into the current collecting metal electrode, the voltageto be applied to the carbon fibers differs from that applied to thecarrent collecting metal electrode due to the contact resistance of thecarbon fibers. This can be detected by the phenomenon that the voltagereturns immediately to the initial state When the application of voltageis stepped, in other words, the increase in so-called overvoltage, andso on. Still further, such problem occurs that, when the surface area ofthe electrode is increased, the difference in potential at the pointsfar from the current collecting metal electrode becomes large due to theresistance of the carbon fibers and, as the result, uniform doping anddedoping hardly occurs.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of an embodiment of the electrodeaccording to the present invention, in which an electroconducting foilis applied to a carbon fiber sheet.

FIG. 2 is a schematic illustration of another embodiment of theelectrode according to the present invention, in which anelectroconducting foil is applied to a carbon fiber sheet.

FIG. 3 is a schematic illustration of another embodiment of theelectrode according to the present invention, in which electroconductingwires are arranged in carbon fibers in the parallel direction to thefibers.

FIG. 4 is a schematic illustration of another embodiment of theelectrode according to the present invention, in which the carbon fibersare woven in networks of electroconducting wires.

In FIGS. 1 to 4, 1 stands for a carbon fiber, 2 for a electroconductingwire, 3 for the direction of the taking-out electrode, 4 for a carbonfiber sheet, and 5 for an electroconducting wire, respectively.

BEST MODE FOR CARRYING OUT OF THE INVENTION

In order to solve the problems described above, the present invention isconstructed as the followings.

That is the first invention of the present application is to provide anelectrode comprising an uni-directionally arranged body of carbon fibersand a secondary battery using the same. In the present invention, theform in which the carbon fibers are arranged in an uni-axial directioncan give excellent packed density and handling property of the carbonfibers. In this case, it is preferable that the carbon fibers are placeduniformly. If there exist some uneven areas in the carbon fiberarrangement, uniform doping can sometimes not be given.

The electrode according to the second invention of the presentapplication is characterized by comprising a carbon fiber sheet and afoil or wires having electrical conductivity.

The practical modes preferably employed in the present invention areillustrated concretely in the followings with reference to the drawingsattached.

FIG. 1 illustrates an embodiment of the electrode of the presentinvention, in which carbon fibers are arranged in an uni-axial directionand an electroconducting foil is applied thereto. FIG. 3 illustratesanother embodiment of the electrode of the present invention, in whichcarbon fibers are arranged in an uni-axial direction andelectroconducting wires are also arranged in the same direction as thecarbon fibers. In FIG. 3, 1 stands for carbon fibers, 2 forelectroconducting wires represented by metal fibers, and 3 for thedirection connecting with the taking-out electrode, respectively. In thecase where the carbon fibers are arranged in an uni-axial direction asshown in these drawings, by arranging the electroconducting wires in thedirection vertical to the direction of the carbon fibers so that thecarbon fibers are bound with the electroconducting wires, the carbonfibers come to be fixed to some extent; which is the more preferablemode for practice. In addition, as shown in FIG. 4, by weaving thecarbon fibers arranged in an uni-axial direction into the networks ofelectroconducting wire, not only the carbon fibers can be prevented frombecoming into pieces, but also the electrical collection efficiencybecomes well.

In order to improve the electrical conductivity with the carbon fibers,a method in which a sheet-shaped carbon fibers arranged in an uni-axialdirection are closely adhered on an electroconducting foil representedby a metal foil is preferably employed. This method can be carried out,for example, by adhering a part or all of the carbon fibers on a metalfoil under compression by means of roll press and the like, or adheringthe carbon fibers on a metal foil using a small amount of a resin suchas Teflon and poly(vinylidene fluoride) as a binder.

In case where the electrode is rolled up, the direction of the carbonfibers to be arranged is preferably approximately vertical against therolled direction. This is because that, this arrangement can prevent theloosening of the carbon fibers placed inside of the metal foil and canmake the carbon fibers to be hardly broken when the electrode is rolledup. Furthermore, by such arrangement, there can also be prevented thepenetration of the broken carbon fiber edges through a separator or thesticking of the broken carbon fiber edges out of the both ends of theelectrode by moving in zigzag directions. The penetration through aseparator and sticking out of the both ends of the electrode of thebroken carbon fiber edges are undesirable since they may cause theelectrical short circuit with the positive electrode.

As described above, the electrode in which the carbon fibers areintegrated with the metal foil enables to lower the contact resistanceof the carbon fibers and to make the distance between the metalcollector and the carbon fibers shorter and, as the result, more uniformpotential in the carbon fibers can be given. Therefore, the decrease incapacitance caused by overvoltage resulting from the contact resistanceand the non-uniform doping caused by the non-uniform potential in thecarbon fibers can also be prevented.

The weight of the carbon fibers to be arranged in an uni-axial directionis preferably not smaller than 30 g/m² and not larger than 200 g/m², andmore preferably not smaller than 50 g/m² and not more larger than 150g/m². When the weight is too large, the carbon fiber sheet itselfbecomes thick and the resistance of the thickness direction becomeshigh, which results in occasional non-uniform doping and difficulty inuse of the resulting electrode at high output current. On the otherhand, when the weight is too small, the amount ratio of the carbonfibers, i.e. active material, based on the whole amount of the resultingnegative electrode becomes small, which results in decrease in theamount of the carbon fibers to be packed in a battery and reduction inenergy density of the battery.

The carbon fiber to be used in the present invention is not particularlylimited, but a filament prepared by firing an organic substance isgenerally used. Specific examples of such carbon fiber include aPAN-based carbon fiber prepared from polyacrylonitrile (PAN), apitch-based carbon fiber prepared from pitch of coal, petroleum or thelike, a cellulose-based carbon fiber prepared from cellulose and a vaporphase grown carbon fiber prepared from gas of a low molecular organicmaterial. In addition, other carbon fibers prepared by firing poly(vinylalcohol), lignin, poly(vinyl chloride), a polyamide, a polyimide, aphenol resin, furfuryl alcohol and so on can also be employed. Thecarbon fiber to be used is suitably selected from those listed abovedepending on the intended properties of the resulting electrode orbattery.

Among the carbon fibers listed above, when used for a negative electrodeof a secondary battery in which a nonaqueous electrolytic solutioncontaining an alkali metal salt is used, preferably employed are aPAN-based carbon fiber, a pitch-based carbon fiber and a vapor phasegrown carbon fiber. In particular, from the viewpoint of a good dopingproperty with lithium ions, a PAN-based carbon fiber is most preferable.

In the present invention, the carbon fibers obtained by firing may besubjected to any subsequent treatment and any type of carbon fiber maybe employed, so long as it retains a form of carbon fiber. Inparticular, the carbon fiber which is subjected to thecharging/discharging treatment in an electrolytic solution prior toincorporating into a battery are effectively used since it can reducethe initial capacity loss (i.e. retention) inherent to a carbonaceousmaterial. The initial capacity loss results from the phenomenon that apart of dopants (e.g. lithium ion) which are doped during the initialcharging step remains in the carbonaceous material and the residue isnot dedoped in the subsequent discharging step. In order to improve thecapacity of a secondary battery, it is effective to reduce the initialcapacity loss. The carbon fiber itself has an electrical conductivityand is a continuous material, and therefore suitable for the previouscharging/discharging treatment. Specific example of such previoustreatment method is that in which the carbon fiber is doped or dedopedin an electrolytic solution containing lithium ions.

In the second invention of the present application, the form or shape ofthe carbon fiber sheet is not particularly limited, but is preferably asheet-shaped structural in which the carbon fibers are arranged in anuni-axial direction. In the cloth-type or felt-type carbon fiber sheet,any form may be employed such as woven fabric, knit fabric, plaitedfabric, lace, mesh, felt, paper, non-woven fabric and mat.

The diameter of the carbon fiber to be used in the present inventionshould be determined so that the carbon fibers can be prepared in theform as described above, and preferably 1 to 500 μm, more preferably 3to 10 μm. It is also preferable to use several kinds of carbon fibershaving different diameters individually.

As the metal to be used as an electrically conductive foil and wire,there can be employed gold, silver, copper, platinum, rhodium, aluminum,iron, nickel, chromium, manganese, lead, zinc, tungsten, titanium, andso on. In addition, alloys of the metals listed above can also beemployed, such as stainless steel. These metals may be coated of theirsurfaces with various substances so long as they are impaired of theirelectrical conductivity. The metal or coated one thereof is made into afoil or a wire, and then arranged with the carbon fibers in the formsshown in the drawings. In case of metal foil, a thin foil is preferablyused, since the thick metal foil causes to decrease in the amount of theactive material to be stored in a battery. The thickness of the metalfoil is preferably about 5 to 100 μm. In particular, from the viewpointsof electric resistance and thickness and cost of the metal foil to beused, copper foil is preferably employed. On the other hand, in case ofmetal wire, the diameter should be determined depending on properties,diameter and shape of the carbon fiber used, so that the currentcollecting effect is enhanced or the carbon fibers are bundled easily,but is preferably about 1 to 200 μm, more preferably 5 to 100 μm. Inorder to increase the intensity of bundling of carbon fibers, it ispreferable to bundle several fine metal wires in a twisted form.

The ratio between the carbon fibers and the electroconducting wires inan electrode of the present invention should be determined suitablytaking into consideration of the properties and current collectingefficiency of the resulting electrode and so on. However, the ratio ofthe electroconducting wires to the carbon fibers of the resultingelectrode is preferably 1 to 10% by weight and 0.2 to 2% by volume, andmore preferably 2 to 8% by weight and 0.4 to 2% by volume.

In the fibrous and cloth-shaped carbon fibers, the partial breaking ofthe single fiber in the carbon fiber bundle, i.e. fuzzing, tends tooccur. The fluffs sometimes penetrate through a separator to contactwith a positive electrode, which causes the internal short circuit. Inorder to prevent this defect, it is effectively carried out to paste andcoat a part or all of carbon fibers with a resin. The resin to be usedis not particularly limited, and a conventional thermoplastic orthermosetting resin can be employed. In particular, preferably used area fluororesin, an olefin resin, an epoxy resin, an urethane resin, anacryl resin and a polyester resin singly or in combination thereof, anda modified one thereof.

The method for pasting and coating carbon fibers with a resin is notparticularly limited. However, it is preferable to paste and coat carbonfibers by passing the carbon fibers through a polymer solution oremulsion vessel, or by spraying the solution or emulsion thereon. Whenthe amount of the polymer to be coated on the carbon fibers is toosmall, the fuzzing of the carbon fibers can not be depressedsufficiently. On the other hand, when the amount of the polymer is toolarge, the function of the carbon fibers themselves as active materialtends to be reduced.

From these reasons, the amount of the polymer used for coating of carbonfibers is preferably not less than 0.1 part by weight and not more than15 parts by weight based on 100 parts by weight of the carbon fibers.When the amount is less than 0.1 part by weight, the fuzzing can not beprevented sufficiently. On the other hand, when the amount is over 15parts by weight, the electrical properties of the carbon fibers asnegative electrode active material is affected. In particular, when thedischarging current becomes over 500 mA per 1 g of the negativeelectrode active material, the initial discharge capacity tends to beresuced to 70% of that given when the carbon fibers are uncoated.

From these reasons, the coating amount of a polymer is most preferably0.5 to 10 parts by weight, and particularly 0.5 to 8 parts by weight. Inthe method for coating a polymer on carbon fibers, in the case where thepolymer is solved in an water soluble organic solvent such asN-methylpyrrolidone, it is more effective to precipitate the polymer bywet solidification in water or a mixed solution of an organic solventand water.

The separator to be used in the present invention is not particularlylimited, and a commercially available product can also be employed, solong as it is a porous film, a woven fabric, a non-woven fabric and soon having insulating property, such as that made of polyolefin,polypropylene, polytetrafluoroethylene, polyethylene and polyacetal. Thefilm thickness of the separator is preferably not larger than 200 μm,and more preferably not larger than 50 μm, for the purpose of reducingthe internal resistance of the resulting battery. More specifically,"Cellguard" (a trade name produced by Daicel kabushiki Kaisha) and"Highpore" (a trade name produced by Asahi Kasei Kogyo Kabushiki Kaisha)are preferably used.

As the material used for a positive electrode as a constituent of asecondary battery, a carbon fiber can be used. In addition, there canalso be used artificial or natural graphite, carbon fluoride, aninorganic compound such as a metal and a metal oxide, and an organichigh molecular compound. When an inorganic compound such as a metal anda metal oxide is used for a positive electrode, the charging/dischargingreaction occurs utilizing the phenomenon of doping and dedoping ofcations. On the other hand, when an organic high molecular compound isused for a positive electrode, the charging/discharging reaction occursutilizing the phenomenon of doping and dedoping of anions. Thus, thecharging/discharging reaction takes various manners according to thekinds of the substances employed, and is suitable selected according tothe intended properties of the positive electrode of the resultingbattery.

Specific examples of the positive electrode material include inorganiccompounds such as oxides and chalcogenides of transition metalsinvolving alkali metals; conjugated polymers such as polyacetylene,poly(para-phenylene), poly(phenylene vinylene), polyaniline, polypyrroleand polythiophene; bridged polymers having disulfide bond(s); thionylchloride; and so on; which are compounds used in conventional secondarybatteries. Among these, in case of a secondary battery using anonaqueous electrolytic solution containing lithium ions, an oxides orchalcogenide of a transition metal such as cobalt, manganese,molybdenum, vanadium, chromium, iron, copper or titanium is preferablyused. In particular, compounds LiCoO₂ and LiNiO₂ are most preferablesince they exhibit high voltage and large energy density.

The electrolytic solution to be used for the secondary battery in whichthe electrode of the present invention is used is not particularlylimited, and a conventional one is employed such as an acidic oralkaline aqueous solution or non-aqueous solvent. In particular, as anelectrolytic solution for a secondary battery using a non-aqueouselectrolytic solution containing an alkali metal salt listed above,there are preferably used propylene carbonate, ethylene carbonate,τ-butyrolactone, N-methylpyrrolidone, acetonitrile,N,N-dimethylformamide, dimethylsulfoxide, tetrahydrofuran,1,3-dioxolane, methyl formate, sulfolane, oxazoline, thinyl chloride,1,2-dimethoxyethane, diethylene carbonate, derivatives thereof andmixtures of two or more of them. As the electrolyte to be contained inthe electrolytic solution, are preferably employed halides of alkalimetals, especially of lithium, perchlorates, thicyanates, borofluorides,phosphofluorides, arsenofluorides, aluminofluorides,trifluoromethylsulfates, and so on.

For the application to a secondary battery which uses a non-aqueouselectrolytic solution containing an alkali metal salt, the electrodecomprising carbon fibers of the present invention utilizes a phenomenonof doping of cations or anions to carbon fibers. Therefore, thiselectrode can be used for both of negative and positive electrodes, andpreferably for a negative electrode of a secondary battery. Inparticular, when cations represented by lithium ions are doped, thecarbon fibers show excellent properties as a negative electrode materialfor high energy-type battery which exhibits high capacity and basepotential. In addition, since the carbon fibers are used in a fibrousform, they can make their contact resistance lower compared with carbonpowder, and as the result high current discharge becomes possible.

The secondary battery using the electrode of the present invention canbe applied to various portable small electronic devices such as videocameras, personal computers, word processors, radios with cassette,portable telephones and so on, due to its characteristics oflightweight, high capacitance and high energy density.

EXAMPLES

The present invention will be illustrated in more detail with referenceto the following examples; however, these examples are intended to beunderstood not to limit the scope of the present invention.

Example 1

A tow-shaped structural body containing 12000 carbon fibers was made bybundling "TORAYCA T300" (a trade name produced by Toray Industries,Inc.) carbon fiber, which was fixed at its ends with anelectroconductive copper paste. Five of the resulting tow-shapedstructural bodies are arranged unidirectionally, and then adhered oftheir ends with copper foils, to give a sheet of 20 mm in length, 50 mmin width and about 0.3 mm in thickness. The weight of the resultingcarbon fiber sheet was 230 mg.

For the determination of the capacitance of the uni-directionallyarranged body of the carbon fibers for a secondary battery, atriode-type cell was prepared using lithium foils as a counter electrodeand a reference electrode and using a solution in which 1M of lithiumperchlorate had been dissolved in propylene carbonate as an electrolyticsolution. The resulting cell was charged until the voltage reached to 0V at a constant current of 20 mA, and after resting for 20 minutes, wasdischarged until the voltage reached to 1.5 V at a constant current of20 mA to determine the discharge capacity. As the result, thecapacitance of the cell per weight was 304 mAh/g, and it was proved thatthis method could give a high discharge capacity.

Example 2

(1) Preparation of electrode

On a copper foil of 15 μm in thick, commercially available PAN-basedcarbon fibers "TORAYCA T-300" (a trade name produced by TorayIndustries, Inc.) were placed in an uni-axial direction uniformly, togive an electrode comprising a copper foil and carbon fibers, in whichthe weight of the carbon fibers was 100 g/m².

(2) Preparation of positive electrode

Commercially available lithium carbonate (Li₂ CO₃) and basic cobaltcarbonate (2CoCO₃.Co(OH)₂) were weighed so that the molar ratio of thesecomponents became Li/Co=1/1, and then mixed with each other using a ballmill. The resulting mixture was treated by heating at 900° C. for 20hours, to give LiCoO₂. The resulting LiCoO₂ was crushed using a ballmill. A slurry for a positive electrode was prepared by mixing theLiCoO₂, artificial graphite as an electroconducting material,poly(vinylidene fluoride) (PVDF) as a binder and N-methylpyrrolidone asa solvent in a mixing ratio of LiCoO₂ /artificial graphite/PVDF=80/15/5by weight. The resulting slurry was applied on an aluminum foil, driedand pressed; whereby a positive electrode was given.

(3) Preparation of battery

Two kinds of electrodes prepared in steps (1) and (2) above,respectively, were superposed upon each other with interposing aseparator of a porous polypropylene film, ("Cellguard #2500"; a tradename by Daicel Kagaku Kabushiki Kaisha) therebetween, and then rolledup, to give a cylindrical electrode body. The resulting electrode bodywas immersed into a beaker cell in which an electrolytic solution ofpropylene carbonate containing 1M of lithium perchlorate was put.Terminals were taken out from the copper foil and the aluminum foil,respectively; thus a secondary battery was prepared.

(4) Evaluation of battery

The secondary battery thus prepared was evaluated for its chargingproperty. That is, the secondary battery was charged until the voltagereached to 4.3 V at a constant current of 40 mA/g as the current densityper weight of the carbon fibers, and was discharged. The dischargecapacity of the secondary battery, which was determined from the chargeamount given by the discharging, was 320 mAh/g per weight of the carbonfibers used in this battery.

Example 3

As the carbon fibers, commercially available PAN-based carbon fibers("TORAYCA T-300"; a trade name produced by Toray Industries, Inc.; 3K;3000 fibers) were used. As the polymer for pasting and coating of thecarbon fibers, a commercially available poly(vinylidene fluoride) resin("Neoflon VP-850"; a trade name produced by Daikin Kagaku KabushikiKaisha) was used by dissolving in N-methyl-2-pyrrolidone.

The carbon fibers were immersed in the PVDF solution, and then furtherimmersed in a solution having a composition of water:N-methyl-2-pyrrolidone=1:1 (by weight) to solidify the polymer. Theresultant was dried at 150° C. for 1 hour, to give carbon fibers pastedwith the polymer. The PVDF polymer-adhered carbon fibers thus preparedhad an adhesion amount of the polymer of 5% by weight based on theweight of the carbon fibers and an average pore diameter measured usinga SEM photograph of about 15 μm.

In order to examine the influence of fluffs of the carbon fibers pastedand coated with the polymer against a separator, the carbon fibers isinserted into polypropylene porous film ("Cellguard"; a trade nameproduced by Daicel Kagaku Kabushiki Kaisha) to be fixed and then woundup around a stainless steel rod. The resultant was applied with a linepressure of 2 kg/cm for 10 minutes to observe whether the carbon fiberspenetrated through a separator or not. As the result, no penetration ofcarbon fibers was observed.

According to the same method as Example 2, a triode-type beaker cell wasprepared using these carbon fibers woven into networks of nickel finewires as a working electrode, metal lithium as a counter electrode and areference electrode, and 1M-LiClO₄ /propylene carbonate as anelectrolytic solution. The resulting cell was charged (i.e. doped withlithium ions) until the voltage reached to 0 V (vs Li⁺ /Li) for thereference electrode at a constant current of 100 mM/g based on theweight of the carbon fibers, and after resting for 20 minutes,discharged (i.e. dedoped) under the same condition until the voltagereached to 1.5 V (vs Li⁺ /Li); thus the cell was charged and dischargedto determine the discharge capacity.

As the result, the discharge capacity was 350 mAh/g, which was the samevale as that given when the carbon fibers were not pasted or coated withPVDF polymer. Accordingly, the reduction in discharge capacity caused bythe pasting and coating with a polymer was not recognized.

On the other hand, when the carbon fibers were used as they were withoutpasting or coating with a polymer, the discharge capacity given was 351mAh/g. However, when the penetration of the carbon fibers through aseparator was examined, 10 to 15 of penetrating ponts by the carbonfibers were observed on the separator.

Example 4

(1) Preparation of electrode

20 mg of commercially available PAN-based carbon fibers "TORAYCA T-300"(a trade name produced by Toray Industries, Inc.) were arranged in anuni-axial direction, and woven their ends with nickel fine wires(diameter: 100 μm) in the direction vertical to the arranged directionof the carbon fibers, and bundled as shown in FIG. 3; thus an electrodewas prepared. In the resulting electrode, the weight ratio between thecarbon fibers and the metal fine wires was 100:1.

(2) Evaluation of charging property

The evaluation of charging property was carried out using the electrodeprepared above. In this evaluation, a triode-type liquid cell was usedin which propylene carbonate containing 1M lithium perchlorate was usedas an electrolytic solution and metal lithium foils were used as acounter electrode and a reference electrode, respectively, and theliquid cell was charged until the voltage reached to 0 V (vs Li⁺ /Li) ata constant current of 40 mA/g as the current density per weight of thecarbon fibers. As the result, the return of the voltage, i.e.overvoltage, given after resting for 20 minutes was 10 mV.

Example 5

(1) Preparation of electrode

20 mg of commercially available PAN-based carbon fibers "TORAYCA T-300"(a trade name produced by Toray Industries, Inc.) were arranged in anuni-axial direction, and woven their ends with nickel fine wires(diameter: 100 μm) in a network form as shown in FIG. 4; thus anelectrode was prepared. In the resulting electrode, the weight ratiobetween the carbon fibers and the metal fine wires was 100:1.

(2) Evaluation of charging property

The evaluation of charging property was carried out using the electrodeprepared above in the same manner as Example 1. As the result, theovervoltage given after charging was 0.5 mV.

Example 6

A coin-shaped secondary battery was prepared using the carbon fibers ofExample 3, which had been pasted and coated with a PVDF polymer, as anegative electrode, the LiCoO₂ /artificial graphite/PVDF of Example 2 asa positive electrode; in a manner that the positive electrode and thenegative electrode were superposed upon each other with interposing aseparator. In this secondary battery, 1M-LiClO₄ /propylene carbonate wasused as an electrolytic solution.

The charging/discharging test was carried out using 100 pieces of thecoin-shaped secondary batteries prepared above. As the result, nodefective such as short circuit was not observed, and all of thesecondary batteries tested were operated normally.

Example 7

(1) Preparation of carbon fiber electrode and charging/dischargingthereof

A beaker cell was prepared using commercially available PAN-based carbonfibers "TORAYCA M40" (a trade name produced by Toray Industries, Inc.)bundled with Ni wire of a current collector as a working electrode, andusing metal lithium as a counter electrode and a reference electrode,and using 1M-LiClO₄ /propylene carbonate as an electrolytic solution.

In the resulting cell, lithium ions were doped until the voltage reachedto 0 V (vs. Li⁺ /Li) to the reference electrode and then dedoped underthe same condition until the voltage reached to 1.5 V (vs. Li⁺ /Li);thus the charging and discharging of the cell was completed.

(2) Preparation of secondary battery and evaluation thereof

The carbon fibers which had been charged and discharged previously inthe step (1) above were arranged on a Ni mesh. The resultant wassuperposed upon the positive electrode prepared in the same manner asExample 4 with interposing a separator; thus a coin-shaped cell wasprepared. In this cell, propylene carbonate containing 1M lithiumperchlorate was used as an electrolytic solution. When this cell wascharged and discharged, the Coulomb s efficiency given was 96%. In thiscell, by the previous charging and discharging, the initial volume losswas, reduced from the value of 30 mAh/g given when no treatment wascarried out to the value 5 mAh/g.

Industrial Applicability

As described above, the electrode of the present invention comprises anuni-directionally arranged body of carbon fibers or one comprising thecarbon fibers and electrically conductive foil or wires. By using theelectrode for a chargeable/dischargeable secondary battery in which acarbonaceous material capable of doping and dedoping of lithium ions isused as a negative electrode active material, there can be provided asecondary battery having high capacitance and high outputting property.

We claim:
 1. An electrode which comprises a sheet of uni-directionallyarranged carbon fibers extending be essentially in a single direction,which sheet is placed on a metal foil, wherein the electrode is rolledup so that the arranged direction of the carbon fibers is approximatelyperpendicular to the rolling direction of the electrode.
 2. Theelectrode according to claim 1, wherein the metal foil is a copper foil.3. The electrode according to claim 1, wherein electroconducting wiresare further arranged in the carbon fibers in a direction parallel to theaxial direction of the fibers or in a direction perpendicular to theaxial direction of the fibers.
 4. The electrode according to claim 1,wherein the carbon fibers are pasted and coated with a resin.
 5. Theelectrode according to claim 4, wherein the resin is a thermoplasticresin.
 6. The electrode according to claim 4, wherein the resin is athermosetting resin.
 7. The electrode according to claim 4, wherein theamount of the resin used is not smaller than 3% by weight and not largerthan 17% by weight based on the amount of the carbon fibers.
 8. Theelectrode according to claim 4, wherein the amount of the resin used isnot smaller than 5% by weight and not larger than 10% by weight based onthe amount of the carbon fibers.
 9. The electrode according to claim 4,wherein the resin is poly(vinylidene fluoride).
 10. The electrodeaccording to claim 1, wherein the carbon fibers which have beenpreviously charged and discharged are used as active material.
 11. Theelectrode according to claim 1, which is used as a negative electrode.12. A secondary battery which uses the electrode according to claim 1.13. The secondary battery according to claim 12 which uses a non-aqueouselectrolytic solution containing lithium salt and a positive electrodecapable of taking in and out of lithium.
 14. The secondary batteryaccording to claim 12, wherein a transition metal oxide is used for apositive electrode.
 15. The secondary battery according to claim 12,wherein the transition metal oxide is LiCoO₂ or LiNiO₂.
 16. An electrodewhich comprises a sheet of uni-directionally arranged carbon fibersextending essentially in a single direction, which sheet is placed on ametal foil, wherein the electrode is rolled up so that the arrangeddirection of the carbon fibers is approximately parallel to the rolleddirection of the electrode.